The present invention relates to a head positioning control system applicable to a disk storage drive such as a hard disk drive (HDD) or the like, and particularly, to a head positioning control system (or servo-system) using a MR (magnetoresistive) head as a read head.
Conventionally, in a disk storage drive such as a HDD or the like, a magnetic head is used to write data on a disk as a storage medium and to read data from a disk. Other than a magnetic disk drive such as a HDD, a magneto-optical disk may be considered as a disk drive. As a practical example of the present invention, a HDD is cited in the following explanation.
In recent years, in order to realize a higher recording density, a head of a read/write separation type using a MR (magnetoresistive) head as a read head has been put to use. This type of head has a structure in which a read head (MR head) and a write head (inductive head) are equipped on a slider as a main body of the head.
In a HDD, when making access with specified data in a disk (to read or write data), operation for reading servo data previously recorded on a disk is read by a read head. A CPU of the HDD uses the servo data read by the read head, to make positioning control by which the position of the read head on the disk is detected and the read head is moved to a target position to be accessed. The target position to be accessed means a track (or cylinder) specified for reading or writing data. In writing operation, the write head is positioned at a specified track, by the positioning control of the read head.
Head positioning control made by the CPU can be roughly divided into seek control (or speed control) for moving a head to a target track and track following control (or fine position control) for positioning the head to a certain range of the target track. The seek control uses a track address (or cylinder code) contained in servo data. Meanwhile, the position control (or track following control) uses servo burst data (a, b, c, and d) described later. The present invention relates to position control using servo burst data.
In the following, position control in a servo system (or head positioning control system) of a HDD will be briefly explained with reference to FIGS. 8 and 9.
As shown in FIG. 8, a number of coaxial tracks (N-1, N, N+1, are provided in a disk 1. In FIG. 8, the range from a position X1 to a position X5 corresponds to the range of a track N. A servo area is provided at a same position in the radial direction in each of the tracks. Servo areas are arranged with an equal intervals inserted between each other in the circumferential direction of the disk 1. Servo data including a cylinder code described above and servo burst data is previously recorded in the servo area (by a specialized servo writer when manufacturing a HDD). FIG. 8 shows only servo burst data (a, b, c, and d). In the following, burst patterns of items of the servo data are expressed as small letters a to d, and amplitude values (of digital values which will be explained later) reproduced from the burst pattern items are expressed as burst data items A to D. In each of the tracks, a data sector (which is an area for recording user data) is provided between the servo areas.
Servo burst data consists of burst patterns a and b arranged at right angles to the track center (which is the center X3 of the track N) as a reference, and burst patterns c and d arranged at right angles to a boundary position (X5 or X1) as a reference.
The servo system (CPU) makes seek control to move a read head 3 to the vicinity of a specified track (N), and then, sample-holds amplitude values corresponding to burst patterns a to d read out by the read head 3. The CPU inputs position data items A to D obtained by converting the amplitude values into digital values. Further, the CPU executes calculation processing (or positional displacement calculation) for detecting the position (or positional displacement from a reference position) of the read head 3 with use of the burst data items A to D obtained.
Specifically, when the read head 3 is positioned in the vicinity of the center (X3) of the track N, as shown in FIG. 8, the CPU inputs position data items A and B corresponding to burst pattern items a and b, and executes calculation of "PI=(A-B)/(A+B)". Position information PI calculated by the calculation is equivalent to a positional displacement amount E of the read head 3 from the center position X3 of the track N as a reference. The CPU executes position controls of the read head 3 so that the position displacement amount E is eliminated to zero.
Meanwhile, as shown in FIG. 8, as a HDD has come to have a higher recording density, the head width of the read head 3 has been decreased. Therefore, the relationship between the head width TW and a track pitch (or track width) satisfies "TP&gt;TW". This track width TP is a track range for positioning the read head 3. When the head width TW and the track width TP thus satisfy the relationship of "TP&gt;TW", the read head 3 is out of the range of the burst pattern b if the read head is in the vicinity of the position X5, and therefore, the burst data B is always zero. Therefore, the CPU cannot obtain proper position information PI from the calculation "PI=(A-B)/(A+B)". As for the CPU the vicinity of the position X5 in the track range is an insensible band in detection of the position of the read head 3. Therefore, the CPU executes calculation of "PI=(C-D)/(C+D)", using burst data items C and D corresponding to burst patterns c and d, to calculate a positional displacement amount E of the read head 3 from the track boundary position X5 (as shown in FIGS. 9 and 10). In FIGS. 9 and 10, the lateral axis denotes a head position (HP9) and the longitudinal axis denotes position information (or a position displacement) PI. Also, the continuous line 100 indicates position information based on a result of the calculation "PI=(A-B)/(A+B)" and the broken line 101 indicates position information based on a result of the calculation "PI=(C-D)/(C+D)". In FIG. 10, the broken line 101 indicates position information including a shift amount (+1) equivalent to half of one track.
As has been described above, since the relationship between the head width TW and the track width TP satisfies "TP&gt;TW", the calculated position information PI is greater than the actual shift amount (E) of the read head 3, and this was confirmed from experiments. Hence, the CPU sets a position-conversion factor K and makes a correction by multiplying the position information by the factor K. Note that the position-conversion factor K is a fixed value set by TP and TW. EQU PI={(A-B)/(A+B)}*K (1)
In correction of position information with respect to the position X5 as a reference, position information PI is naturally obtained by multiplying a result of "(C-D)/(C+D)" by the factor K. By improving the position conversion factor K, the CPU is capable of obtaining position information PI having a so-called linearity. In FIG. 11, the continuous line 111 indicates a calculation result of the calculation formula (1), and the broken line 112 indicates position information obtained by taking into consideration a shift amount (+0.5 track) of a half track.
As described above, in a servo system of a HDD, the CPU executes calculation processing related to position information for detecting a position of a read head (or position displacement amount from a reference position), with use servo burst data. In the calculation processing, the CPU uses a predetermined position factor K to calculate position information having a linearity. As a result of this, the CPU is capable of detecting accurately the position of a head from position information having a linearity, and of controlling positioning of a head with respect to a target position (which is normally the center position of a track range).
However, as described above, a MR head used as a read head in a HDD in recent years has a high read sensitivity so that this head can output a high reproduce level but the high read sensitivity is unstable. Specifically, as shown in FIG. 13, the output signal amplitude S is not uniform with respect to a read gap RG (equivalent to the head width of the read head). Therefore, the read head 3 has an output characteristic that levels are different between positions in the left and right sides of the read gap RG. Therefore, when the CPU calculates position information with use of servo burst data read out by the read head 3, position information having a linearity cannot be obtained with respect to a head position HP, as indicated by a continuous line 120 in FIG. 12. Note that a broken line 121 in FIG. 12 indicates a position information characteristic having an ideal linearity in position control of a servo system.
Meanwhile, as a HDD has come to have a larger storage capacity, a track pitch of a disk and a read gap RG of a read head have been decreased to be smaller, while the non-uniformity of the read sensitivity has been relatively enhanced in the read gap RG. In addition, due to manufacturing factors of a MR head, dispersion of the read sensitivity occurs frequently so that it is considered difficult to attain a uniform output sensitivity in the near future.
Hence, a position information characteristic obtained from the read head 3 is adjusted based on the read sensitivity characteristic of a MR head, as shown in FIG. 15. In FIGS. 14 and 15, contihuos lines 70 and 71 indicate calculation results concerning position information, using burst data items A and B, in case where a read head 3 is positioned in the vicinity of the center X3 of a track N. Continuos lines 70a and 71a respectively indicate position information characteristics each having an ideal linearity. Broken lines 80 and 81 indicate calculation results concerning position information, using burst data items C and D. Continuos lines 80a and 81a respectively indicate position information characteristics each having an ideal linearity. However, as shown in FIG. 15, the position information characteristic includes a non-linearity when the read head 3 is positioned in the vicinity of a position X4, even if the CPU adjusts the position information characteristics, based on the read sensitivity characteristic of the MR head.
Further, in practical use, recording of servo data onto an actual HDD is influenced by an external vibration or a limitation concerning pitch feed of a servo write. As a result, serve data recorded on the disk 1 already includes an error in comparison with an ideal state. Consequently, a problem occurs in that position information obtained by one of two pairs of burst data items a and B and c and d do not perfectly correspond to the other one of the pair. This causes a factor which deteriorates performance seek control, i.e., position control in the vicinity of the connecting point between them. An example of this problem is shown in FIG. 16. FIG. 16 indicates that the position information (characteristic) 72 corresponding the pair of burst data items A and B and that corresponding to the position information (characteristic) 82 are not continuous to each other when the read head 3 is positioned in the vicinity of the position X4. Note that position information characteristics 72a and 82a are respectively position information characteristics having an ideal linearity.